1. Field of the Invention
The invention relates to an optical head, and more particularly to an optical head recording data signals to and reproducing recorded data out of an optical recording medium such as a compact disk including substrates having different thicknesses.
2. Description of the Related Art
A widely used optical disk includes an optical head which transmits a light through a transparent substrate to thereby form a micro-spot on a plane of a recording medium in order to protect the plane fabricated in micron-order.
In such a conventional optical head, a light emitted from a laser source inside the optical head is transmitted as a converging beam through a transparent substrate comprising a parallel plate of an optical disk. Thus, the beam is caused to have wave aberration which is dependent on a thickness of the parallel plate through which the beam has passed. Herein, wave aberration indicates a gap between an equiphase of a converging beam and a spherical plane. A great gap causes a converging spot to expand beyond the diffraction limit, resulting in that it is impossible to obtain good data-reproduction performance. Hence, a lens is designed to compensate for the wave aberration in an optical head system in order to form a micro-spot in the vicinity of the diffraction limit on a plane of a recording medium.
In general, an optical disk is designed to have a different thickness in accordance with what it is applied to. The compensation for the above mentioned wave aberration varies in dependence on a thickness of an optical disk. Hence, a conventional optical head has a problem that it can reproduce only data recorded in an optical disk having a specific thickness.
There has been suggested another optical head which utilizes a transmitted zero order light and a +1st order diffracted light derived from a holographic optical element (HOE) in order to be able to use two disks having different thicknesses (Y. Komma, S. Nishino and S. Mizuno, "Dual Focus Optical Head for 0.6 mm and 1.2 mm Disks", Optical Review, Vol. 1, No. 1, 1994, pp. 27-29).
The suggested optical head is partially illustrated in FIG. 1. In the illustrated optical head, a beam emitted from a laser source (not illustrated) passes through a compensation holographic optical element 20 and is focused onto both a first optical disk 5 having a thickness of 0.6 mm and a second optical disk 6 having a thickness of 1.2 mm by means of an objective lens 4. Lights reflected at the first and second optical disks 5 and 6 pass through the objective lens 4 and then the compensation holographic optical element 20, and are received in a photodetector (not illustrated).
The objective lens 4 is designed to be adapted for the first optical disk 5 having a thickness of 0.6 mm and reproduce data-recorded signals derived from the first optical disk 5 by means of a transmitted zero order light derived from the compensation holographic optical element 20 and having no phase changes. In order to adapt for the second optical disk 6 having a thickness of 1.2 mm, the compensation holographic optical element 20 is designed to have a grating to emit a +1st order diffracted light for compensating for aberration still existing in a light transmitting through the objective lens 4 due to a difference in a thickness between the first and second optical disks.
However, since a beam is to be divided into pieces by the compensation holographic optical element 20 in the above mentioned conventional optical head, the conventional head poses a problem that it is not avoidable that an efficiency of utilizing a light is reduced, and accordingly an amount of light is significantly reduced. For instance, provided that a division ratio is 50% for a transmitted zero order light and also 50% for a +1st order diffracted light, the efficiency of a light is merely 25%, because the light passes through the compensation holographic optical element 20 twice in outgoing and incoming paths.